That's what Reddit founder Alexis Ohanian learned when he started dating the tennis superstar, according to a new Vanity Fair profile on the couple.

Williams and Ohanian met in 2015 and are planning to get married later this year. The couple is also expecting their first child this fall.

Williams is arguably one of the most successful tennis players, if not the most successful athlete, of her generation: She's racked up 23 grand slams and more than $84 million in prize money, according to Vanity Fair.

But Ohanian didn't pay much attention to tennis before he met her, and had no idea how tough she was. He considered himself a hard worker and thought Silicon Valley techies put in more time and dedication than anyone else.

Here's how Vanity Fair puts it:

Alexis knew then he wanted to marry her, not simply out of happiness or compatibility. She was helping him become the best version of himself because of her own work ethic and focus, with millions watching and the expectation of the public that she should win every time, what Serena herself described as carrying "three pyramids" on her shoulder. He thought he worked hard — it is part of the romance of high tech that everyone works 18 hours a day and then curls up under the desk for a few hours' sleep with their laptop as teddy bear and pacifier — but he realized it was nothing compared with Serena.

Don't believe it? According to Vanity Fair, the week before the Australian Open, Williams was concerned about missing too many backhands, so she hit 2,500 of them over the course of nearly three hours and if she missed one, she started over. Oh, and she was pregnant at the time.

Case closed.

You can read more about how Ohanian and Williams met and fell in love over at Vanity Fair.